Galvanized steel and process of making same



Patented Feb. 2, 1943 7 GALVANIZED STEEL AND rRocEss OF MAKING SAIVIE Nicholas P. Veeder, St. Louis, Mo., assignor to Granite City Steel Company, Inc., Granite City, L, a corporation of Delaware No Drawing. Application June 27, 1941 Serial No. 400,101

3 Claims.

Hot-dipped galvanized steel may be conceived as consisting of three layers, namely, an inner layer or base composed of steel, an outer layer or-coating composed of zinc and an intermediate or bonding layer consisting of the materials of the other two layers bonded or alloyed together. Unless the bond between the zinc coating and the steel base is sufficiently strong, the zinc will peel or flake off when the. galvanized steel is bent abruptly as in the processes of beading or curling or whenit is submitted to press-working. The principal object of the present invention is to insure such tight bonding of the zinc coat to the steel basethat the product will stand abrupt bending and press-working without substantial peeling or flaking. l a

The invention consists in the processor producing galvanized steel hereinafter described and claimed.

According to the present invention, 1 use for a base a steel containing an appreciable proportion of silicon as an alloying element. Specific examples of steel suitable for the purpose are stated hereinafter.

In the manufacture of galvanized steel sheets according to my invention, the process is as follows: Starting with a steel slab with a suitable content of silicon, the same is hot-rolled in a four-high mill into strip and the strip is pickled and washed to remove the iron oxides therefrom and is then galvanized by hot-dipping for about ten minutes in a bath of zinc at a temperature gfabout 840 Fahrenheit. The product thus produced has a coating with far greater adherence to the base metal thanpthat of galvanized sheet steel produced by similarly treating steel sheets whose silicon content is less than .12 per cent. V

The following is aspeciflc example that illusustrates my invention, starting with a slab of the following composition: carbon .08 per cent, manganese .30 per cent, phosphorusv .007 per cent, sulphur .029 per cent, copper .202 per cent,

silicon .128 per cent, the balance being substantially all iron. This slab was heated and delivered to the roughing stands of a four-high strip rolling mill of live stands at a temperature of about 2250 degrees Fahrenheit and was reduced in a single heat to strip of a thickness of approximately .075 inch. This heavy reduction of the strip from .420 inch to .075 inch by the live stands of the four-high mill involved severe working of themetal and brought about a high degree of preferred orientation of the iron crystallites thereof and this preferred orientation remained after the'strip cooled. The strip was not annealed but was cut into sheets, and pickled, without annealing, by passing it through a bath of 10 per cent sulfuric acid in which all parts of the sheets were immersed for a period of about thirty minutes. Thereafter the sheets were promptly rinsed with water and then hot-dipped in a bath of molten zinc at a temperature of about 840 degrees Fahrenheit, with which salammoniae was used as a flux in the usual manner of galvanizing.

'The following are other examples of steels that have been treated in: the way pointed out in connection with the first example, namely:

For Example No. 2, the slab was of the following composition, namely: carbon .13 per cent, manganese .46 per cent, phosphorus .006 per cent, sulphur .034 per cent, copper .075 per cent.-

silicon .14 per cent, the balance being substantially .all iron. This'slab was heated and delivered to the roughing stands of a four-high I strip rolling mill of five stands at a temperature of about 2250 degrees Fahrenheit and was reduced in a single heat to strip of a thickness of approximately .075 inch. Then it was coiled, pickled in a bath of about 10 per cent sulphuric acid, rinsed, oiled, and cold reduced to a thickness of.about .024 inch. It was then cut into sheets, box annealed for about twenty hours at around 1200 degrees Fahrenheit, temper-rolled,

pickled in 10 per cent sulphuric acid for'about ten minutes, washed and then hot-dipped -galvanized in the usual manner. It is noted that, as the result ofthe severe working of the metal, especially in the cold rolling operation, the strip, when it leaves the mill has the physical properties characteristic of strain. hardening. These properties include a degree of brittleness, which isobjection'able because itunflts the metal for uses that require abrupt bending,. and theyalso include the preferred orientation of the crystallites which is desirable but is liable to be--de- 'stroyed by high temperature annealing. By-annealing at about 1200 degrees Fahrenheit, which is below the critical point of steel of the composition specified, the sheet became ductile but still preserved the preferred orientation of its crystallites characteristic of strain hardened or severely worked steel.

For Example No. 3, a slab of the following composition was used, namely: carbon .12 per cent, manganese .35 per cent, phosphorus .009 per cent, sulphur .026 per cent, copper .144 per cent, 7

- grees Fahrenheit and was reduced in a single heat to strip of a thickness of approximately .075 inch. Then it was coiled, pickled in a bath of about 10 per cent sulphuric acid, rinsed, oiled, and

cold reduced to a thickness of about .024 inch. It was then re-coiled, box annealed forabout twenty hours, deoxidized, temper-rolled, cutinto sheets, and galvanized without pickling.

The foregoing examples, which are typical of numerous others, produced galvanized sheets which were beautifully spangled and could be sharply bent without causing any appreciable peeling or flaking of the zinc. In fact, many of them were submitted to beading and pressworking operations without any apparent impairment of the bond between the base metal and the zinc coating.

While the superior adherence of the zinc coating of my product is readily observable by comparative tests with other hot-dipped galvanized sheets, it is not easy to account for such superiority. The probable explanation is that, in applicants base metal, the greater part of the silicon forms an alloy with the iron, that the severe strain imposed by the four-high rolling mill has the effect of breaking down the initially relatively large grains of the base metal and bringing about preferred orientation of th crystallites, and that the preferred orientation thus brought about and which would be lost or impaired by high temperature annealing is preserved by avoiding high temperature annealing, when annealing is needed to prevent brittleness, and provides a surface that is exceptionally well adapted for hot-dipping. In the case of hot-rolling with a heavy draft, annealing is unnecessary, but in the case of cold rolling, annealing is done at a temperature below the critical point of the base metal, say, at about 1180 degrees Fahrenheit. It is believed that the silicon alloy steel rolled and treated as hereinbefore described so as to have preferred orientation of the crystallites dissolves more slowly in zinc than does the same steel differently rolled and treated; and it has been found that when steel of this particular composition possessing these particular characteristics is hotdipped galvanized, it takes a coating with an exceptionally strong adherence to the base metal. A distinction is noted between the presence of silicon in the steel 'as an alloying element thereof and the mere presence of silicon in the steel as an intrusion or as a residue of the silicon introduced as a deoxidizing or purifying agent in the process of manufacture.- According to the present invention, silicon is used as an alloying element. Accordingly, the silicon is added to the steel while it is in the molten state, and part of'it is consumed in deoxidizing the steel and part remains to alloy with the iron. Steel made under these condition gives satisfactory results in my process even when the total content of silicon is as low as .12 per cent.

It is noted that all of the steels of the foregoing examples had a carbon content low-enough for a ductile steel; and when the galvanized steel is intended to be sharply bent, as by beading or pressworking, its steel base should be low in carbon, say, less than .25 per cent carbon. v Likewise, the manganese content of the several examples ranged from about .27 per cent to about .50 per cent. Within this range the manganese does not appreciably aflect the ductilityof the base metal, whereas a manganese content in excess oi .70 per cent has an adverse effect on ductility and increases the cost or the metal base without any adequate compensation for the additional amount of manganese. The content of copper in the examples specified ranged from .67 per cent to .246 per cent but does not appear to be a significant factor in the matter of bonding'the zinc coat to the steel base. On the other hand, the content of silicon in the metal base is quite significant when present in excess of .12 per cent. When the amount of silicon exceeds 1.5 percent. it tends to impair the ductility of the metal base: and when it is present in smaller quantity than .12 per cent, its effect in improving the adherence of the zinc to the metal base is minimized. For

practical and economical, reasons, I prefer to use a silicon content ranging from about .12 per cent to about .30 per cent.

Galvanized steel sheets made in accordance with my invention are remarkably welladapted for shaping by bending or pressworking and will, without appreciable injury, undergo bending and pressing operations that would work serious injury on sheets coated by the ordinary hot-dipping process.

WhatIclaimis:

1. The improvement in the process of manu, facturing galvanized sheet steel which consists in producing a slab of low carbon steel containing more than .12 per cent but less than 1.5-per cent of silicon, converting said slab into a ductile sheet having the preferred orientation of its iron crystallites characteristic of severely worked steel, and dipping said sheet in molten zinc.

2. The improvement in the process of manufacturing galvanized sheet steel which consists in producing a slab of low carbon steel containing more than .12 per cent but less than 1.5 per cent of silicon, reducing said slab by cold rolling into a sheet having preferred orientation of its iron crystallites, annealing said sheet at a tem perature of approximately 1200'degrees Fahrenheit to make said sheet ductile without destroying said preferred orientation of its iron crystallites, and dipping said sheet in molten zinc.

3. The improvement in the process of manufacturing galvanized sheet steel which consists in cold-rolling under high pressure a strip of steel containing upwards of twelve hundredths of one per cent but less than one and one-half per cent of silicon to form a sheet, having preferred orientation of its crystallites, annealing NICHOLAS P. VEEDER. 

